Inner ear mechanosensory hair cells convert mechanical vibrations into electrical signals via the coordinated interaction of multiple proteins precisely positioned within the sensory hair bundle. In ongoing studies we have been studying the time course for the acquisition and maturation of mechanoelectric transduction (MET) in rat cochlea outer hair cells (OHC) maintained in organotypic cultures. A spatio-temporal developmental progression was observed morphologically and functionally with basal cochlea maturation preceding apical cochlea by 2-3 days in all measured properties. The fraction of mechanosensitive cells increased rapidly, with a midpoint at post natal day 0 (P0) for basal cells, and correlating with myosin IIIa immunoreactivity. MET current magnitude increased over several days. Adaptation lagged the onset of transduction by a day and matured more slowly, overlapping but preceding the rise in myosin Ic immunoreactivity. Less than 25% of myosin Ic expression was required for the mature adaptation response, suggesting multiple roles for this protein in hair bundle function. Directional sensitivity, lacking in immature responses, developed rapidly and correlated with the pruning of radial links and an increase in tenting of stereociliary tips. Morphological and electrophysiological data support a hypothesis where key elements arrive independently to the site of MET, with a mature response occurring as membrane tension increases, likely by the increased tensioning of the tip-link with the onset of adaptation. Organotypic cultures developed normal, tonotopically specific, MET response properties, suggesting maturation was not influenced significantly by external factors such as innervation, endolymph, normal mechanical stimulation, or an intact organ of Corti. [unreadable] [unreadable] We have also been studying the composition and properties of the tip link apparatus. In the widely accepted model of mechanoelectrical transduction (MET), deflection of the stereocilia bundle increases tension in a gating spring that raises the open probability of the MET channels located near the tips of stereocilia. The tip-link has been suggested as the gating spring because it connects stereocilia tips along the direction of mechanosensitivity. We have now demonstrated that tip-links are formed by the interaction of two members of the cadherin super-family of Ca2+-dependent adhesion molecules. Using immunogold labeling we showed that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) form the upper and lower portions of the tip-link respectively. The first cadherin repeats of CDH23 and PCDH15 co-localize along the tip-link filament suggesting that these proteins interact end-to-end to form an asymmetric cross bridge. Ca2+ removal, which causes disruption of tip-links and a relaxation movement in the bundle, causes an upward movement of CDH23 immunolabeling confirming the Ca2+-dependence of the CDH23-PCDH15 complex and tension in the tip-link. Based on this proposed molecular composition of tip-links, we are now testing current theories about the role of tip-links, the processes of acquisition and maintenance of mechanosensitivity in hair cells, and the potential underlying mechanisms of hearing loss associated with mutations in CDH23 and PCDH15.